Conventionally, as a driving source of an acoustic component such as a speaker, an electromagnetic actuator, owing to its easy handling, is widely used. The electromagnetic actuator is made up of a permanent magnet and a voice coil and vibration is generated by an action of a magnetic circuit of a stator using the permanent magnet. In an electromagnetic speaker using the electromagnetic actuator as a vibration source, a sound is produced by a vibration plate having low stiffness such as an organic film, which is connected to a vibration section of the electromagnetic actuator in a fixed manner. In recent years, demands for mobile phones, laptop personal computers, or a like are increasing, which also causes a gradually increased demand for a small-sized and power-saving type actuator. However, in the case of the electromagnetic actuator, it is necessary to allow large amounts of current to flow through a voice coil at time of operation, which causes a problem to occur in terms of power consumption and its configuration to be unsuitable structurally for achieving miniaturization and thinning. Additionally, in the electromagnetic actuator, in order to prevent a failure caused by a magnetic field leaked from the voice coil, magnetic shielding is required for application to an electronic device, which causes its configuration to be unsuitable for application to a small-sized electronic device such as a mobile phone. Furthermore, the miniaturization of the electromagnetic actuator causes a wiring material making up the voice coil to become small in size and, as a result, a resistance value of the wiring material increases, which produces a possibility of overheating of the voice coil.
To solve the above problems, as a thin-type vibration component other than the electromagnetic actuator, a piezoelectric actuator using a piezoelectric element such as piezoelectric ceramic as a driving source is being developed which has features of being small-sized, lightweight, power-thrifty, and non-leakage of the magnetic field. The conventional piezoelectric actuator is so configured as to produce mechanical vibration by movements of a piezoelectric element and has a structure in which the piezoelectric ceramic element (hereinafter, simply referred to as a piezoelectric element) is bonded to a pedestal.
By referring to FIGS. 57 and 58, basic configurations of the piezoelectric actuator manufactured according to related art are described below. FIG. 57 is a perspective view of the piezoelectric actuator according to the related art. FIG. 58 is a cross-sectional view schematically showing a mode of vibration occurring in the piezoelectric actuator in FIG. 57. As shown in FIG. 57, the piezoelectric actuator 550 includes the piezoelectric element 510 made of a piezoelectric ceramic, a pedestal 524 to which the piezoelectric element 510 is fixed, and a frame-like support member 527 to support a peripheral portion of the pedestal 524. By configuring as above, the piezoelectric element 510, when an AC current is applied thereto, performs an expanding and contracting movement. As shown in FIG. 58, the pedestal 524 is deformed, according to the expanding and contracting movements described above, into a convex mode shape (shown by a solid line) or into a concave mode shape (shown by a broken line) by using junction portions 524a at both sides of the pedestal 524 as their fulcrums. Thus, the pedestal 524 vibrates in up and down directions as shown in FIG. 58 using the junction portion 524a at both sides of the pedestal 524 as fixed ends and using the central portion of the pedestal as an antinode.
The piezoelectric actuator generally has a feature of being advantageous to achieving its thinning, however, also another aspect of being inferior in acoustic performance when compared with the electromagnetic actuator. This is attributable to the fact that, though a large amplitude can be obtained at a frequency close to a resonant frequency when compared with the case of the electromagnetic actuator owing to high stiffness and a high mechanical Q value (mechanical quality coefficient) of the piezoelectric element itself, the amplitude at a frequency other than the resonant frequency is small. Moreover, when the amplitude of vibration of the piezoelectric actuator is small, the sound pressure becomes also low, which means that sufficient sound pressure cannot be obtained in a wide frequency band (range) required for reproduction of music or the like. Also, the conventional piezoelectric actuator presents a problem that the occurrence of divided vibration caused by intrinsic resonance (high-order intrinsic resonance in particular) determined by its outer shape and stiffness of its materials causes a lowering of flatness of frequency characteristics of sound pressure level.
The divided vibration described here is a phenomenon in which the piezoelectric actuator, when vibrating, vibrates in a region divided at a local node of vibration at a non-uniform phase at an intrinsic resonant frequency. In other words, this is the phenomenon in which the vibration plate of the piezoelectric actuator does not vibrate in an integral manner as a whole, but vibrates partially in a random manner. FIG. 59 is a concept diagram showing the state of the occurrence of divided vibration in the piezoelectric actuator. Moreover, FIG. 60 is a schematic diagram to explain the state of divided vibration. That is, as shown in FIG. 60, when divided vibration occurs in the vibration plate of the piezoelectric actuator, a node of vibration occurs at the midpoint between fixed points at both sides and one vibration plate vibrates at a non-uniform phase. The occurrence of the divided vibration of this kind causes various inconveniences that sound other than an acoustic signal inputted into the piezoelectric actuator is produced and that sound pressure attenuates by phase interference in vibration regions (that is, divided region) whose phases differ from one another and, furthermore, peaks and dips (that is, hills and valleys) occur, thus causing a lowering of flatness of sound pressure level frequency characteristics, thus resulting in degradation of acoustic characteristics.
As technology to solve problems as described above, a piezoelectric actuator has been disclosed (for example, Patent References 1 and 2) in which a peripheral portion of a pedestal is so configured as to be supported by a beam which is relatively easily deformable to increase the vibration amplitude of the actuator. For the same purpose, another technology has been also disclosed (for example, Patent Reference 3) in which a plate spring having slits inserted along a circumference of the peripheral portion of a pedestal is fabricated to obtain a large vibration amplitude. Still another technology has been disclosed (Patent Reference 4) in which a peripheral portion of a pedestal is bonded to a support member with a bending-type supporter being interposed between the pedestal and support member and the range of frequency characteristics is made broad.    Patent Reference 1: Japanese Patent Application Laid-open No. Sho61-168971    Patent Reference 2: Japanese Patent Application Laid-open No. 2000-140759    Patent Reference 3: Japanese Patent Application Laid-open No. 2001-017917    Patent Reference 4: Japanese Patent Application Laid-open No. 2001-339791